Experimental characterisation of charge distribution and transport in electron irradiated PMMA

For studying the space charge distribution and transport in poly-methyl-methacrylate (PMMA) polymer material, the specimens were submitted to electron irradiation using a scanning electron microscope (SEM). An arrangement adapted to the SEM for measuring the leakage and displacement currents have been used. Measurements help in estimating the charge quantity and studying trapping processes during and after the electron irradiation. The properties of the dynamic trapping of PMMA and the time constants of charge processes were evaluated. Many physical processes are involved in the charging and discharging mechanisms; among them surface conduction is outlined. The effective resistivity of the sample is measured and compared with that of classic and standardized methods. The strength of the electric field initiating surface discharge is also estimated.     

The charge transfer and ion formation in liquid Li–Tl alloys

The ⁷Li NMR Knight shift, K, and the spin-lattice relaxation time, T₁, were measured for liquid Li–Tl alloys. The K decreases rapidly with the addition of Tl up to 20 at.% Tl. In the concentration from 20 to 50 at.% Tl, the K decreases only slightly and the K of 50 at.% Tl is 60% of K for the pure liquid Li. Such a decrease of K is considered as an indication for the strong charge transfer from Li to Tl. These tendencies are similar to those from previous studies for liquid Li–Ga and Li–In alloys. However, beyond 50 at.% Tl, the K increases and reaches to an almost constant value (70% of K for the pure liquid Li). Such a back donation of charge is absent for liquid Li–Ga and Li–In alloys. It is considered that the tendency of the formation of ionic structural unit for liquid Li–Tl alloys is slightly weaker compared with the cases of liquid Li–Ga and Li–In alloys. The T₁ is also discussed with the relation to the Knight shift and the electronic properties.     

Intermediates and transport phenomena in two‐temperature synthesis of ZnGeP2

High quality semiconducting ternary compound ZnGeP₂ was synthesized by a modified two‐temperature technique using high purity elemental zinc, germanium and phosphorus as the starting materials. Transport phenomena of zinc and phosphorus vapors and the major reaction intermediates, taking place in ZnGeP₂ formation, were studied by interrupting the synthesis process using quenching technique as well as by adjusting the temperatures of cold and hot zones. The powder X‐ray diffraction analysis showed that the major reaction intermediates were ZnP₂, Zn₃P₂, and GeP, which proportions were changed at the different temperature stages. ZnP₂ was formed in the temperature gradient region and ZnGeP₂ was formed in the hot zone when the temperature of the hot zone was higher than 900 °C. The 520‐1040 °C temperature profile was chosen for the ZnGeP₂ synthesis and charge amount per run reached 200 g. The powder X‐ray diffraction pattern of the synthesized ZnGeP₂ compound was in agreement with the standard pattern of ZnGeP₂. These results demonstrated that the synthesized ZnGeP₂ compound was a single phase. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lattice dynamics and ion transport in structurally disordered chalcogenides of copper and silver

The results of studying the copper and silver selenides and their solid solutions are reported. The methods of inelastic neutron scattering and neutron diffraction analysis are used to study the dynamics and structure of the crystal lattice. The relation of the properties of dynamics and the crystal-lattice structure to the ion transport are analyzed.     

Predicted structure,thermo-mechanical properties and Li ion transport in LiAlF4 glass

Materials with the LiAlF₄ composition are of interest as protective electrode coatings in Li ion battery applications due to their high cationic conductivity. Here classical molecular dynamics calculations are used to produce amorphous model structures by simulating a quench from the molten state. These are analysed in terms of their individual pair correlation functions and atomic coordination environments. This indicates that amorphous LiAlF₄ is formed of a network of corner sharing AlF₆ octahedra. Li ions are distributed within this network, primarily associated with non-bridging fluorine atoms. The nature of the octahedral network is further analysed through intra‐ and interpolyhedral bond angle distributions and the relative populations of bridging and non-bridging fluorine ions are calculated. Network topology is considered through the use of ring statistics, which indicates that, although topologically well connected, LiAlF₄ contains an appreciable number of corner‐linked branch‐like AlF₆ chains. Thermal expansion values are determined above and below the predicted glass transition temperature of 1340 K. Finally, movement of Li ions within the network is examined with predictions of the mean squared displacements, diffusion coefficients and Li ion activation energy. Different regimes for lithium ion movement are identified, with both diffusive and sessile Li ions observed. For migrating ions, a typical trajectory is illustrated and discussed in terms of a hopping mechanism for Li transport.     

Surface chemistry and transport effects in GaN hydride vapor phase epitaxy

A simple quasi-thermodynamic model of surface chemistry in GaN hydride vapor phase epitaxy (HVPE) is presented. The model is coupled with the detailed 3D simulations of species transport in a horizontal-tube reactor and validated by the comparison with the data on the GaN growth rate obtained by laser reflectometry. Parametric study of the growth rate as a function of temperature and species flow rates has been performed over a wide range of growth conditions. The important role of species transport in an HVPE reactor is demonstrated. In particular, a strong effect of the natural concentration convection resulting in the formation of recirculation zones and in a non-uniform vapor composition is revealed by modeling. The impact of these effects on the GaN growth rate and V/III ratio on the growth surface is discussed in detail.     

Study of ion transport behavior in a mechanochemically synthesized silver halide mixed composite system: [0.75 AgI:0.25 AgCl]

Ion transport behavior in a mechanochemically synthesized silver halide mixed composite has been investigated. AgI and AgCl, in 75: 25 mol. wt. (%) ratio, has been prepared by high energy ball milling for different milling times viz. 5, 10, 20, 30, 40, and 50 h as well as by simple physical mixing (referred to as zero hour milling). The room temperature conductivity has been evaluated as a function of milling time. The conductivity increased initially with increasing milling time, then decreased. The increase in conductivity has been attributed to the increased degree of amorphosity in the sample as a consequence of high energy ball milling. The 30-hour milled sample exhibited highest conductivity with an enhancement of more an order of magnitude from that of zero hour milled sample. To explain the ion transport mechanism in the optimum conducting sample, other basic ionic parameters viz. ionic mobility (μ), mobile ion concentration (n), ionic transference number (t_ion) etc. have also been determined. The materials and structural properties have been characterized using XRD, FTIR, and DSC techniques. Using the optimum conducting sample as electrolyte, all-solid-state battery has been fabricated having cell configuration: Ag (anode) // (0.75 AgI:0.25AgCl) // C + I₂ + Electrolyte (cathode), and the cell performance has been studied under varying load conditions.     

Temperature, H2 loading and ultra violet irradiation effects in germanosilicate optical fibers: laser spectroscopy measurements

The effects of heat-treatment and ultraviolet (UV) light at 325 nm on virgin and hydrogen loaded germanosilicate optical fibers have been investigated by using laser spectroscopy technique. The luminescence band around 650 nm is sensitive to heat treatment. The hydrogen reloading of the hydrogen loaded fibers subsequent to heat treatment showed a new emission band around 570 nm. The irradiated fibers have an emission band around 383 nm which was not detected in hydrogen loaded fibers.     

Photo-induced phenomena in chalcogenide glass: Comparison with those in oxide glass and polymer

Two topics, the both being related with localized states in non-crystalline solids, are studied. One is a comparison of photo-induced phenomena in oxide, chalcogenide, and organic materials. Despite of different inorganic and organic structures, there exist many similarities in photo-induced phenomena, which can be ascribed to excitation of localized electronic states. The other topic concerns photo-induced phenomena induced by linear and non-linear excitation. A result on As₂S₃ demonstrates that band-gap excitation by one- and two-photon processes provides different changes. It is suggested that the two-photon process occurs resonantly at around localized states in amorphous materials, and the process plays important roles in the structural change.     

Ion transport behavior in a new fast Ag ion conducting composite electrolyte system: 0.85[0.75AgI:0.25AgCl]:0.15CeO2

Investigations on ion transport behavior of a new fast Ag⁺ ion conducting composite electrolyte system: 0.85[0.75AgI:0.25AgCl]: 0.1CeO₂ are reported. An alternate host: ‘[0.75AgI:0.25AgCl] mixed system/solid solution’ has been used as first-phase host matrix salt, in place of the traditional host AgI, while the micron-size particles of an insulating and chemically inert CeO₂ as second-phase dispersoid. The soaking time, plays important role in determining the conductivity enhancement in the composite system. The system: 0.85[0.75AgI:0.25AgCl]:0.1CeO₂ prepared at soaking time ∼10 min. exhibited optimum conductivity:σ_rt ∼ 1.2 × 10⁻³ S cm⁻¹ at room temperature, which is an order of magnitude higher than that of the pure host. Structural characterization studies were performed by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Temperature dependent measurement on the basic ionic parameters viz. conductivity (σ), ionic mobility (μ), mobile ion concentration (n), room temperature ionic transference number (t_ion) and ionic drift velocity (v_d) have been carried out on the system.     

Playing with nanosegregation in discotic crown ethers: from molecular design to OFETs,nanofibers and luminescent materials

Discotic liquid crystals are promising materials for electronic device applications. Combining extended π-systems with crown ethers may further open access to functional supramolecular hybrid materials. In the present publication, we briefly summarise recent literature developments and focus in a second part on our work in the field of discotic crown ethers. The crown ether derivatives are conveniently accessible by convergent syntheses which allow a high degree of variation with respect to side chains, mesogenic unit, size and symmetry of the crown core, metal complexation and counterion. This set of compounds provides a basis for systematic structure–property relationship investigations. Numerous experimental studies identified factors being crucial for mesophase stabilisation and geometry and furthermore led to novel room-temperature crown ether mesogens. The latter were obtained by congestion of the mesogenic units and even introduction of peripheral δ-methyl branched side chains. The formation of molybdenum clustomesogens and their application as luminescent hybrid materials is also described.     

Dielectric spectroscopy of low-loss sugar lyophiles: II. Relaxation mechanisms in freeze-dried lactose and lactose monohydrate

This article presents an analysis of the dielectric relaxation mechanisms of the disaccharide lactose, in both the freeze-dried state and the crystalline monohydrate. Complex dielectric permittivity spectra for both forms of lactose were measured over wide ranges of frequency (0.1 Hz to 1 MHz) and temperature (−120 °C to +100 °C). The crystalline monohydrate displays a sharp peak in the temperature domain which was ascribed to the percolation of migrating protons over the surfaces of the crystals. The freeze-dried material displayed three relaxation processes. The first (γ) and second (β) processes were observed at low temperature and present as very broad relaxations with low dielectric strength. The relaxation times for both processes showed Arrhenius temperature-dependencies, with activation energies of 52 kJ/mol and 72 kJ/mol, respectively. The mechanisms of dielectric relaxation were ascribed to the motion of the pendant hydroxymethyl group and the relative motion of the pair of saccharide rings, respectively. Analysis and discussion of the third relaxation process is beyond of the scope of this article.     

Magnetic resonances of Fe and Ni nanoparticles in films of silicon suboxide produced by ion irradiation of triethoxysilane gels containing Fe or Ni solute atoms

The structure and magnetic properties of composite films obtained by ion irradiation or thermal treatment of triethoxysilane gels containing iron or nickel in solution are investigated by means of X-ray diffraction, electron microscopy and electron spin resonance. Ion irradiated gels are converted into silica glasses containing metallic nanoparticles with a narrow range of sizes. These particles exhibit a magnetic resonance with a preferential alignment of magnetic moments perpendicular to the surface. This unusual out of plane anisotropy seem to be ascribed to an interaction of the moments with radiation defects. The magnetization in irradiated film is comparable to that in films of same materials submitted to a thermal conversion at 1000 °C in vacuum. But the latter films are porous and contain particles of metal and silicide or silicate with a wider range of sizes.     

MeV electron irradiation induced crystallization in metallic glasses: Atomic structure,crystallization mechanism and stability of an amorphous phase under the irradiation

MeV electron irradiation via high voltage electron microscopy can lead to amorphous-to-crystal transition (i.e., crystallization) as well as crystal-to-amorphous transition (i.e., solid-state amorphization). Irradiation-induced crystallization can be observed in various alloy systems such as Co-, Fe-, Ni-, Pd-, and Zr-based metallic glasses, indicating that this phenomenon has wide generality in metallic materials. Irradiation-induced crystallization mechanism was discussed based on the following factors; (1) an increase in free energy for an amorphous phase, (2) the formation of crystalline clusters through modification of the atomic configuration near radiation induced defects, and (3) enhanced diffusion. The stability of an amorphous phase against irradiation-induced crystallization can be estimated from the thermal crystallization temperature (T_x), and Ni–Nb based metallic glasses have a tendency for high stability against irradiation because of high T_x.     

Color centers and charge state change in Ce:YAG crystals grown by temperature gradient techniques

Color centers and impurity defects of Ce:YAG crystals grown in reduction atmosphere by temperature gradient techniques have been investigated by means of gamma irradiation and thermal treatments. Four absorption bands associated with color centers or impurity defects at 235, 255, 294 and 370 nm were observed in as-grown crystals. Changes in optical intensity of the 235 and 370 nm bands after gamma irradiation indicate that they are associated with F⁺-type color center. Charge state change processes of Fe³⁺ impurity and Ce³⁺ ions take place in the irradiation process. The variations of Ce³⁺ ions concentration clearly indicate that Ce⁴⁺ ions exist in Ce:YAG crystals and gamma irradiations could increase the concentration of Ce³⁺ ions. Annealing treatments and the changes in optical density suggest that a heterovalent impurity ion associated with the 294 nm band seems to be present in the crystals.     

Space charge capacitance spectroscopy in amorphous silicon Schottky diodes: Theory,modeling, and experiments

Numerical modeling of capacitance spectroscopy of hydrogenated amorphous silicon Schottky diodes is carried out. We test the accuracy of the determination of the density of states at the Fermi level, g(E_F), from an analytical treatment of the temperature (T) and frequency (f) dependence of the capacitance (C). Assessment of the position of the Fermi level and the attempt-to-escape frequency of states at E_F is also addressed. It is shown that the precision and reliability of the determination of g(E_F) is strongly dependent on the position of the Fermi level and the shape of the DOS and that the attempt-to-escape frequency is overestimated. Numerical calculations are then used to fit experimental capacitance data. Material parameters that provide the best fits are found in quite good agreement with independent modulated photocurrent and constant photocurrent measurements. Again, the attempt-to-escape frequency deduced from the simplified analytical treatment of capacitance data is found to be overestimated.